Jawaharlal Nehru Technological University Hyderabad B.Tech IV Year MINE SUBSIDENCE ENGINEERING Previous Question Paper (March - 2021 - R16/R15/R13)

Geological disturbances such as faults, folds, joints, and dykes significantly alter the stress distribution and rock mass behavior in underground mines, directly impacting convergence. Fault planes act as planes of weakness, reducing the structural integrity of the roof and floor, which leads to increased convergence rates due to block sliding or rotation. In the presence of faults, the redistribution of abutment pressures is often non-uniform, causing localized high-stress zones that accelerate creep and bed separation. Folds introduce varying dip angles and residual tectonic stresses, which can cause unpredictable roof behavior and excessive floor heave. Furthermore, dykes and sills create stiffness contrasts within the strata; when mining approaches these rigid features, the sudden transfer of load onto the coal face or adjacent pillars results in rapid, non-linear convergence. These disturbances interrupt the natural bridging action of the strata, preventing the formation of a stable pressure arch and leading to increased subsidence and convergence compared to undisturbed geological conditions.

In the total extraction method by caving, convergence is a dynamic process driven by the systematic collapse of the goaf (mined-out area) as the face advances. The process follows a distinct sequence:

• Immediate Roof Fall: As the coal is extracted, the immediate roof loses its support and collapses into the goaf. This initial caving provides a measure of support to the overlying strata, limiting the total convergence.
• Main Roof Caving: As the face advances further, the main roof, which acts as a cantilever or beam, eventually exceeds its tensile strength and fails periodically. This creates a rhythmic cycle of convergence known as periodic weighting.
• Strata Movement: The caving process creates a broken zone, a fractured zone, and a continuous deformation zone above the extraction area. Convergence at the face is the cumulative result of the compression of the caved material in the goaf and the bending/sagging of the upper strata layers.
• Goaf Compaction: As the goaf fills with broken rock, it undergoes compaction under the weight of the superincumbent strata. The degree of convergence is inversely proportional to the bulking factor of the caved rock; higher bulking factors result in earlier stabilization and reduced total convergence at the surface.

The total convergence is thus a function of the extraction height, the depth of cover, and the mechanical properties of the roof strata, which dictate the frequency and magnitude of the caving cycles.